The commercial use of Ndxe2x80x94Fexe2x80x94B magnets, particularly in the automobile industry, has been limited because of the susceptibility of such material to corrosion when exposed to a humid environment.
It is known to protect Ndxe2x80x94Fexe2x80x94B magnets against corrosion by using various coating processes based generally on nickel, cadmium and aluminium. However, these coating processes are either too expensive for commercial application or they do not provide adequate long term corrosion protection.
Zinc coating of ferrous-based materials is widely practised. Various procedures are known for this. Hot dipping in molten zinc at about 430xc2x0 C. (galvanising) is known. However, for application to Ndxe2x80x94Fexe2x80x94B magnets, galvanising can cause cracking of the magnets due to thermal shock and also there is poor control over zinc penetration into the magnet, thereby leading to unacceptable variations in the corrosion protection afforded by galvanising.
It is also known to electroplate by cathodic deposition of zinc. However, such electroplating procedure when applied to Ndxe2x80x94Fexe2x80x94B magnets leads to embrittlement of the material by hydrogen absorption.
It is also known to provide a zinc coating by means of the so-called sherardising process, wherein the article to be zinc-coated is tumbled in a rotating barrel containing zinc dust and sand at 380xc2x0 C. However, such a sherardising process when applied to Ndxe2x80x94Fexe2x80x94B materials leads to unacceptable oxidation thereof.
It is an object of said one aspect of the present invention to obviate or mitigate the above disadvantages by providing a process for applying a corrosion-resistant coating on an article which can be performed with reduced risk of cracking and oxidation of the article.
According to said one aspect of the present invention, there is provided a method of applying a corrosion-resistant coating to an article, comprising the steps of embedding the article in a mass of particles containing a sublimable corrosion-resistant material or a precursor thereof, and heating the embedded article at a temperature below the solidus temperature of the corrosion-resistant material under a pressure of less than 65 Pa so as to cause a coherent layer of the corrosion-resistant material to be formed on the article by sublimation.
Also according to said one aspect of the present invention, there is provided an article when coated with a corrosion-resistant coating by the method as defined in the last-preceding paragraph.
The article is preferably a magnet formed, for example, of Ndxe2x80x94Fexe2x80x94B. The pressure during the heating step is preferably not more than about 13.3 Pa(1xc3x9710xe2x88x921 Torr).
The embedding procedure is preferably conducted by introducing the article and the particles into an envelope so that the particles completely surround the article, closing the envelope without sealing it, and then introducing the thus-filled envelope into a vacuum furnace. A getter, such as mischmetal, may be employed to absorb oxygen in the furnace.
When embedding the article in the mass of particles, it is highly preferred for all parts of the article to be embedded substantially uniformly in the particles.
The sublimable corrosion-resistant material may be a sublimable corrosion-resistant metal or alloy. Preferably, the sublimable corrosion-resistant material is zinc, magnesium or cadmium or an alloy of any two or more of these, e.g. a Zn/Mg alloy or a Mg/Cd alloy. The precursor of such material may be one which generates the material under the pressure and temperature conditions prevailing in the furnace. For example, the precursor may be a compound which is reducible to form said sublimable corrosion-resistant material, in which case the mass of particles may include a reducing agent.
The temperature of the furnace depends upon the nature of the sublimable corrosion-resistant material. In the case of zinc, the temperature of the furnace is preferably no higher than 390xc2x0 C., and is more preferably in the range of 350 to 390xc2x0 C., although it is considered that the temperature may be as low as about 250xc2x0 C. provided that the pressure is appropriately low and/or the treatment time is appropriately long. For corrosion resistance purposes, it is preferred to provide a layer thickness of 15-30 xcexcm . For such coatings, at a treatment temperature of 390xc2x0 C. and a pressure of 13.3 Pa, the required thickness can be achieved in about 1 to 2 hours.
For anti-corrosion magnesium coatings, the temperature is typically about 450-500xc2x0 C.
For anti-corrosion cadmium coatings, the temperature is typically about 250-300xc2x0 C.
For alloy coatings with zinc, the temperature is typically derived from the ranges for the alloy ingredients, Thus, for Zn/Cd alloy, the temperature is typically 390-280xc2x0 C. for anti-corrosion coatings.
In the case of zinc, temperatures in excess of 390xc2x0 C. should be preferably avoided because the likelihood of agglomeration of zinc dust/powder on the surface is increased, thereby leading to a less uniform finish. This applies particularly to Ndxe2x80x94Fexe2x80x94B materials.
The particles forming the mass in which the article is embedded preferably comprise a mixture of particles of the sublimable corrosion-resistant material or precursor thereof together with a particles of an inert diluent. Thus, the particles may comprise zinc dust, zinc powder and sand as the particulate inert diluent. The zinc dust typically has a particle size of 5 to 10 xcexcm. The zinc powder typically has a particle size of 50 to 75 xcexcm. The proportions of sand, zinc dust and zinc powder are typically 24:17:3 parts by weight.
The envelope may take the form of a stainless steel foil which is closed by crimping to an extent sufficient to retain the contents therein but not sufficient to seal the envelope hermetically.
Particularly in the case of articles in the form of Ndxe2x80x94Fexe2x80x94B magnets, it may be required, before the embedding step, to prepare the surface, eg by abrading the article gently, e.g. with emery paper, and then cleaning it, e.g. by swabbing, with a hot solvent, e.g. an alcohol such as ethanol. However, it has been found that these surface preparation and cleaning procedures can be avoided by forming a controlled thin layer (0.05 to 1.0 xcexcm) of oxide on the surface of the article, particularly a magnet such as an Ndxe2x80x94Fexe2x80x94B magnet. This is found also to provide a further degree of protection to the underlying magnet.
The method of the present invention has the advantage over sherardising that no rotation of the embedded article in a barrel is required and there is enhanced uniformity of coating and article coverage.
In said another aspect of the present invention, there is provided a method of coating a powder, comprising the steps of mixing the powder with particles of a sublimable material or a precursor thereof, and heating the resultant mixture at a temperature below the solidus temperature of the said particles under a pressure of less than 1xc3x97105 Pa so as to cause a coherent layer of the sublimable material to be formed on the powder by sublimation.
Also according to said another aspect of the present invention, there is provided a powder when coated with a layer by the method as defined in the last-preceding paragraph.
The method according the said another aspect of the present invention is suitable for applying corrosion-resistant coatings (such as those mentioned above in relation to the coating of articles) to powders which are susceptible to oxidative and/or atmospheric corrosion, e.g. Ndxe2x80x94Fexe2x80x94B powders which are formed from the bulk alloy by grinding, or by crushing or hydrogen decrepitating followed by milling, before being formed to the required shape (e.g. by compaction with or without subsequent sintering or by moulding using a resin binder, e.g. PTFE). In the case of resin-bonded articles such as magnets, it is particularly preferred to use PTFE which is effective for excluding oxygen and moisture).
It is also considered possible to form a controlled thin layer oxide layer on the surfaces of powders, eg Ndxe2x80x94Fexe2x80x94B powders, prior to forming the coherent layer thereon, in an analogous manner to that described above for the corrosion protection of articles according to said one aspect of the present invention.
The method according the said another aspect of the present invention is also suitable for the coating of magnetic particles, whether or not they are susceptible to oxidation and/or oxidative corrosion, for the purpose of improving the magnetic properties of magnets formed from such powders. In this respect, coercivity can be improved by the provision of a surface coating on the magnetic particles so as to inhibit the nucleation of reverse magnetic domains on demagnetisation. When a metal such as zinc is used to coat particles of Nd xe2x80x94Fexe2x80x94B, the zinc alloys with free Nd which has migrated to the surface of the particles. Additionally, the coating tends to reduce the surface roughness of the particles and results in particles of improved spheroidal shape which assists in preventing reverse domain nucleation and in improving densification during subsequent compaction of the coated particles to form a densified body.
The sublimable materials and processing techniques referred to above in connection with the coating of articles in accordance with said one aspect of the present invention are also considered to be suitable for the coating of particles in accordance with said another aspect of the present invention. Additionally, articles which have been formed from coated particles produced according to said another aspect may also be coated in accordance with said one aspect to enhance further the corrosion resistance of the article.
With further regard to the coating of powders (especially magnetic powders), it is preferred for the thickness of the coating on the powder to be in the range of about 50 to 100 nm for powders having a particle size in the range of about 3 to 10 xcexcm, which is especially preferred for magnetic powders. This means that the coating need only occupy about 1 to 2 vol % of the total volume of the coated powder.
Zinc coating of powders has been demonstrated using 100 xcexcm radius Ndxe2x80x94Fexe2x80x94B powders. By mixing these powders with zinc powder, zinc dust and sand and heating at 370xc2x0 C. for a process time of 30 minutes, Ndxe2x80x94Fexe2x80x94B powders were uniformly coated with 2.5 xcexcm of zinc. The thus coated magnetic powder is separated from the sand by magnetic separation. Layers of zinc having a thickness of 1 xcexcm or less on Ndxe2x80x94Fexe2x80x94B material are achievable using the same procedure at temperatures in the range 250-300xc2x0 C. e.g. a 1 xcexcm layer of zinc can be grown at 285xc2x0 C. in 2.5 hours.
For Ndxe2x80x94Fexe2x80x94B powders coated with 2.5 xcexcm of zinc, as above, the coercivity of the powder is reduced by around 5% and the remanence is in direct proportion to the amount of Ndxe2x80x94Fexe2x80x94B consumed by the Zn coating. Epoxy bonded magnets fabricated from zinc coated Ndxe2x80x94Fexe2x80x94B powders have shown only slight signs of corrosion after 100 hours autoclave exposure. Identical magnets made from uncoated Ndxe2x80x94Fexe2x80x94B powders disintegrate after 50 hours, or less, under the same test conditions. Precise gravimetric studies have shown that Ndxe2x80x94Fexe2x80x94B powders coated with around 1 xcexcm zinc show no evidence of weight increase, hence oxidation, when heated in air for 60 hours at 200xc2x0 C. Uncoated, but otherwise identical powders show steady weight increments, hence oxidation, with time under the same conditions. Thus, it is considered that the thin zinc coatings make both handling and storage easier.
Slow rotation of the mix at, for example, 5 rpm or less aids uniform powder coating. In connection with the coating of the powder with zinc, for a zinc coating on the powder, at temperatures in the region of 250xc2x0 C., under a pressure of about 13.3 Pa, a coating rate of about 0.1 xcexcm/hr is achievable. Low pressures are preferred to limit oxidation of the powder. For magnesium powder coatings, the temperature may be as low as about 350xc2x0 C. For cadmium powder coatings, the temperature may be as low as 200xc2x0 C.
In the case where coating of magnetic powder is effected using particulate sublimable material or a precursor thereof mixed with a particulate diluent such as sand, the coated particles can be readily separated from the diluent particles by magnetic separation.